Define base error type and use stdlib error wrapping

[meatballhat]

The stdlib error wrapping capability via fmt + %w allows for error wrapping as defined in the Go 2 error wrapping spec (citation needed) and as understood by the third-party errors package (and probably many others). The work to be done is:

• define a const-ish "base" error type for errors that originate in this package
• replace all runtime generated errors (errors.New) with error wrapping via fmt + %w